U.S. Pat. Nos. 4,395,914 and 4,619,144 describe a system in which a gas pressure meter uses a rotor magnetically suspended for free rotation between the electric drive coils in a magnetic field within a tubular housing joinable to a vacuum system. The rotor is driven by the drive coils and maintained above a preset minimum rpm. The mode of operation of such prior art device is based on the pressure-dependent braking (deceleration) of the rotor. Accordingly, speed sensors are arranged in the measuring head of the instrument to detect and respond to the rotational frequency of the rotor, and further to transmit signals to an analyzer, which may be a digital computer. The analyzer analyzes the signals and computes the pressure in the chamber from the deceleration.
It has been found in practice that gas friction vacuum meters of this type show improved stability and resistance to reactive gases compared to other pressure meters used in a high vacuum. Because of this stability, the gas friction vacuum meter is also used as a secondary pressure normal, for example, in an international pressure comparison performed by the Bureau International des Poids et Mesures or used by the Deutsche Kalibrierdienst or also in plasma diagnosis. For such uses it is necessary to transport the gas friction vacuum gauge, for example, between a location of calibration and a location of use. Although changes in measured values amount to less than about 0.5% during careful handling in a laboratory, a change in location may produce changes up to several percentage points. These changes in calibration result from a change of the spherical surface of the rotor of the meter due to chemical processes, for example, due to corrosion in air, or due to mechanical action as a result of the rotor coming in contact with the walls of the tubular housing in which it is accommodated during transportation. This may cause striation or smoothing of the rotor surface. The accuracy of the gas friction vacuum meter as a transfer normal and the accuracy of the transfer of high vacuum pressures as a whole are therefore reduced due to the poor transportation conditions of the rotor of the gas friction vacuum meter.